test_drawing_channels_split: Add an example for channels_split

As channels_split may be difficult to understand, this commit adds a simple
example of its usage.

imgui-examples/examples/test_drawing_channels_split.rs

@@ -0,0 +1,44 @@
+extern crate glium;
+extern crate imgui;
+extern crate imgui_glium_renderer;
+
+mod support;
+
+const CLEAR_COLOR: [f32; 4] = [0.2, 0.2, 0.2, 1.0];
+const WHITE: [f32; 4] = [1.0, 1.0, 1.0, 1.0];
+const RED: [f32; 4] = [1.0, 0.0, 0.0, 1.0];
+
+fn main() {
+    support::run("test_drawing_channels_split".to_owned(), CLEAR_COLOR, |ui| {
+        ui.with_window_draw_list(|draw_list| {
+            // Will draw channel 0 first, then channel 1, whatever the order of
+            // the calls in the code.
+            //
+            // Here, we draw a red line on channel 1 then a white circle on
+            // channel 0. As a result, the red line will always appear on top of
+            // the white circle.
+            draw_list.channels_split(2, |draw_list| {
+                const RADIUS: f32 = 100.0;
+                let canvas_pos = ui.get_cursor_screen_pos();
+                draw_list.channels_set_current(1);
+                draw_list
+                    .add_line(
+                        canvas_pos,
+                        [canvas_pos.0 + RADIUS, canvas_pos.1 + RADIUS],
+                        RED,
+                    )
+                    .thickness(5.0)
+                    .build();
+
+                draw_list.channels_set_current(0);
+                let center = (canvas_pos.0 + RADIUS, canvas_pos.1 + RADIUS);
+                draw_list
+                    .add_circle(center, RADIUS, WHITE)
+                    .thickness(10.0)
+                    .num_segments(50)
+                    .build();
+            });
+        });
+        true
+    });
+}